Keilim, Chapter 6, Mishnah 4. This chapter deals with stoves built out of stones: how they are considered a kli that can receive tumah, and how much of the stone is included in that tumah. The previous mishnah discussed a case where stones were taken away from a stove that had already become tamei. Our mishnah presents the mirror image: stones that are added on to stones which are already tamei.
The mishnah begins: "Shtei avanim she'asa'an kira venitme'u", two stones that a person set up as a kira, a stove, and they then became tamei. The picture is simple. Two stones stand parallel to one another, and a pot is set across the top of them, resting on both, with the fire underneath in the space between. That arrangement turns the pair of stones into a kli, and once tumah reaches them, they are tamei.
In this situation, where the stove consists of nothing more than these two stones, the tumah takes hold of the stones in their entirety. It makes no difference how large the stones are, or how thick and wide they may be. Every part of each stone is tamei.
The mishnah then introduces a change in the arrangement: "vesama samach lazo", he leaned an extra stone up against one of the two, "even achat mikan velazo even achat mikan", one added stone on this side and another added stone against the second one on the other side. Four stones now stand in a line, and between them there are three gaps. A pot can be set over each gap, so what began as a single kira has become three stoves standing next to one another.
What, then, is the din of the original pair, which now occupy the middle of the row of four? Here the mishnah teaches: "chetsyah shelo teme'ah vechetsyah shelo tehorah", each of those stones is split in its status, part of it retaining tumah and part of it clean. Picture one of the middle stones: the side turned toward its old companion is still holding up the central pot, the very kira that received the tumah in the first place, and that side keeps its tumah. Its opposite side, though, has taken on a new job, bearing one of the outer pots together with the stone that was just leaned against it. The stoves that this side serves never contracted tumah at all, and therefore that portion of the stone is tahor.
In other words, the two original stones, which until now were tamei from end to end, have become tamei only in part. The tumah stays with whatever portion of them continues to serve the original stove, while the portion that has been drawn into the service of a new, clean stove is clean.
The mishnah closes with one more turn: "nitelu tehorot, chazru elu letum'atan". If the two tahor stones that were added are removed, the outer stoves cease to exist, and we are left once again with only the two original stones. At that moment those two stones return to their earlier tumah and are tamei in their entirety, exactly as they were at the start.
The logic follows directly from the way tumah attaches to the stones. As long as part of each stone was functioning as the side of a fresh outer stove, that part had a purpose of its own and was not swept up in the tumah of the original kira. Once the added stones are taken away, that outer function disappears. Nothing remains but the single stove formed by the two original stones, and so the tumah of that stove once again claims the whole of each one.